Cartridge-type filter elements in which the filter material comprises a plurality of filter membranes, have been provided heretofore with at least one filter membrane constituted as a prefilter or an upstream filter, with respect to a direction of displacement, and at least one other filter membrane as a final filtering membrane at the downstream side of the filter material with respect to the direction of flow of the liquid medium therethrough.
Filter elements having combinations of membrane filters have been provided heretofore as cartridge filters or as filter disk elements and can be used for the filtration of macromolecular or colloidal solutions, for the removal of particles from water, for the filtration of solvents and for the sterile filtration of frustoconical and biotechnology generated preparations as well as comestibles.
It is known to provide such filter elements, which have two membrane filters as described, so that both filter membranes have pores of the same diameters (so-called homogeneous restriction). Membrane filters are also known in which the prefiltering membrane and the final filtering membrane have different pore sizes (so-called heterogeneous construction).
In all of these known membrane filters, the membrane filters of each element have the same or different pore sizes, either in symmetrical or asymmetrical geometry and the same chemical compositions. The materials used can be nylon 66, cellulose acetate (a cellulose derivative), a polysulfone, polyvinylidenefluoride (PVDF) or polytetrafluoroethylene (PTFE).
The filter elements of the prior art in the homogeneous construction have the disadvantage that the operating life is relatively short since solid particles or colloid materials rapidly plug the pores of the first filter membrane so that the final filtering membrane, which may be only partly loaded, becomes ineffective for filtering purposes since there is no substantial flow therethrough.
With conventional membrane filter elements of the heterogeneous construction, the larger particles are retained by the large pores of the prefiltering membrane while the smaller particles pass the first membrane and are retained on the final filtering membrane because of the smaller pores thereof. In this latter arrangement the useful life of the filter is greater than is the case with filter elements utilizing a homogeneous construction.
Thus filter elements with filter membranes having an asymmetric pore geometry and the same chemical composition in the homogeneous or heterogeneous construction, the useful life of the filter element will be greater than that of filter elements with membranes with symmetrical pores in homogeneous or heterogeneous construction, respectively, but the filtration reliability, namely, the assurance that small microorganisms will be retained on the filter during sterile filtration, is not assured.